The present invention relates to the control of inductive loads by pulse width modulation, and more specifically to a method and a device for this type of control.
The invention is suitable for application, in particular, in the automotive field, for example in electronic systems for controlling the position of the throttle valve (or “ETC” systems, standing for “Electronic Throttle Control”), or in gas recirculation (or “EGR” systems, standing for “Exhaust Gas Recirculation”).
The control of inductive loads by pulse width modulation can be provided by means of an H-bridge structure, comprising four power switches, namely two “high side” switches and two “low side” switches. A first pair, formed by a first high side switch and a first low side switch, causes current to flow in the load in a first direction when these switches are closed. Conversely, a second pair, formed by the other high side switch and the other low side switch, causes current to flow in the load in the other direction. The two low side transistors, or the two high side transistors when they are closed together, causes the flow of what is known as a “freewheeling” current.
Each switch generally comprises a MOS power transistor (acronym for “Metal Oxide Semiconductor”, in other words a field effect transistor with a metal oxide gate). A sequence of analog control signals of the four MOS transistors is provided by using a pulse width modulated set point control signal, the duty cycle of which can be used to control the amount of current injected into the load. Slow variations of current and voltage (called “slew rate”) are created at the control gates of the MOS transistors, in order to avoid sudden switching which would cause electromagnetic disturbance.
Taking the switching sequence into account, the response of the MOS transistors is slower in the opening and in the closing of the switches. Consequently the effective duty cycle of the control signal at the terminals of the load is different from that of the set point control signal. More precisely, it is lower.
This difference is not particularly troublesome in applications in which the load is controlled in a closed loop, since in this case the system is able to adapt so as to provide the desired effective behavior in the load. This is the case, for example, when information relating to the angular position of a shutter controlled by an electric motor is sent by a sensor and is subject to appropriate slaving.
However, there are applications or situations in which the precision of the duty cycle is critical, and sometimes even in a closed loop system when it is desirable for this closed loop to be very fast.
In such a situation, the use of software could be proposed in order to compensate for the error in the duty cycle introduced by slow current and voltage variations (the slew rate), based for example on a table characterizing the behavior of the load and of its analog control means. However, this solution is tedious to implement and does not resolve the problem in all situations.